A very timely article is presented here by Bushe et al., demonstrating a relationship between amyloid plaques and intraneuronal calcium signaling dysregulation using 2-photon imaging techniques in vivo. The authors found diverging populations of neurons in plaque-expressing APPxPS45 mice relative to controls, in that one group decreased calcium transient activity while another “hyperactive” group in close proximity to plaques increased frequency of calcium transients. Reduction in inhibitory synaptic tone resulting from impaired GABAergic transmission is provided as the mechanism.

Overall, this technically demanding study adds to the existing literature demonstrating that neuronal calcium alterations are an integral component of AD pathology—whether through PS mutations, extracellular plaques, or other AD-linked pathways. This is a new view on how AD pathology can affect neuronal signaling, and will hopefully spin off several follow-up studies.

What I do find lacking (perhaps will be addressed in follow-up?) is 1) a mechanism for why GABAergic neurons may be more...  Read more

A very timely article is presented here by Bushe et al., demonstrating a relationship between amyloid plaques and intraneuronal calcium signaling dysregulation using 2-photon imaging techniques in vivo. The authors found diverging populations of neurons in plaque-expressing APPxPS45 mice relative to controls, in that one group decreased calcium transient activity while another “hyperactive” group in close proximity to plaques increased frequency of calcium transients. Reduction in inhibitory synaptic tone resulting from impaired GABAergic transmission is provided as the mechanism.

Overall, this technically demanding study adds to the existing literature demonstrating that neuronal calcium alterations are an integral component of AD pathology—whether through PS mutations, extracellular plaques, or other AD-linked pathways. This is a new view on how AD pathology can affect neuronal signaling, and will hopefully spin off several follow-up studies.

What I do find lacking (perhaps will be addressed in follow-up?) is 1) a mechanism for why GABAergic neurons may be more vulnerable to plaque pathology, since they are loaded with calcium buffers and tend to be less susceptible to excitotoxicity, and 2) a means to determine how/if the “quiet” cells are GABAergic, and if the hyperactive cells are pyramidals or another non-inhibitory other subtype.

Much of cortex contains a granular layer (L. IV) containing largely interneurons, with the exception of the prefrontal region, and it is not clear where in frontal cortex these recordings are taking place. This may change the negative feedback circuit anatomy and have some implications here. Since the pharmacology examines local transmitter effects, it is difficult to make assumptions about the source. The possible role of intracellular calcium waves, triggered by glutamatergic activity but generated by intracellular calcium sources, is also a possible mechanism but is not yet addressed in these studies.

Recently, similar approaches were used in a study from Brian Bacskai's group (Kuchibhotla et al., 2008), and although the general idea is also that calcium levels are disrupted in neurons close to plaques, some significant differences in the details are evident. Although temporal domains of calcium signaling were not the primary focus, it does not appear that calcium oscillations were observed in their cortical neurons. Rather, they observed increases in steady-state calcium levels in neuronal processes close to plaques. Similar to our findings, neurons from control mice do regulate their calcium levels quite well over time (Stutzmann et al., 2006), as do neurons from the AD mice sufficiently removed from plaque deposits. Although mechanisms for this increased steady-state calcium load aren't investigated, it does not appear synaptic/activity-dependent in nature. Granted, there are differences in the techniques and calcium indicator approaches used in the two studies, so as much as I'd like to avoid this euphemism...more studies are needed.

References:
Kuchibhotla KV, Goldman ST, Lattarulo CR, Wu HY, Hyman BT, Bacskai BJ. Abeta plaques lead to aberrant regulation of calcium homeostasis in vivo resulting in structural and functional disruption of neuronal networks. Neuron. 2008 Jul 31;59(2):214-25. Abstract

Stutzmann GE, Smith I, Caccamo A, Oddo S, Laferla FM, Parker I. Enhanced ryanodine receptor recruitment contributes to Ca2+ disruptions in young, adult, and aged Alzheimer's disease mice. J Neurosci. 2006 May 10;26(19):5180-9. Abstract

View all comments by Grace (Beth) Stutzmann

One of the salient outcomes that could reshape our thinking about the role of astrocytes in AD is that calcium signaling alterations linked to dense-core plaque deposits extend beyond the spatial domain of the discrete histopathology, and can synchronize larger populations of astrocyte and astrocyte circuits, either through extracellular signaling or gap junctions. What that calcium is doing, its originating source, and how it affects neurophysiology has yet to be determined in these models.

Certainly, a strength of this study is the confirmation of cell type, as previous in-vivo studies have not done so with certainty and were claiming changes in neuronal calcium signaling, and may have largely been observing astrocytes or other cell types. A potential overinterpretation in this study is relying only on methoxy-X04 staining as an indicator of plaque presence, as this only stains insoluble, late-stage, dense-core deposits and not other perhaps more pathogenic forms such as oligomers and other soluble β amyloid species. In addition, it would be quite interesting to compare...  Read more

One of the salient outcomes that could reshape our thinking about the role of astrocytes in AD is that calcium signaling alterations linked to dense-core plaque deposits extend beyond the spatial domain of the discrete histopathology, and can synchronize larger populations of astrocyte and astrocyte circuits, either through extracellular signaling or gap junctions. What that calcium is doing, its originating source, and how it affects neurophysiology has yet to be determined in these models.

Certainly, a strength of this study is the confirmation of cell type, as previous in-vivo studies have not done so with certainty and were claiming changes in neuronal calcium signaling, and may have largely been observing astrocytes or other cell types. A potential overinterpretation in this study is relying only on methoxy-X04 staining as an indicator of plaque presence, as this only stains insoluble, late-stage, dense-core deposits and not other perhaps more pathogenic forms such as oligomers and other soluble β amyloid species. In addition, it would be quite interesting to compare calcium signaling differences between the APP and APP/PS1 mice.

View all comments by Grace (Beth) Stutzmann

Imaging calcium signals in neural structures like cortex are currently the only way to detect the activity of many or most neurons in a volume of tissue. Clay Reid in the Neurobiology Department at Harvard Medical School did this a while back and made important discoveries about the functioning of visual cortex. The current system, developed by Katsushi Arisaka, is a clever way to improve on the original method that Reid used. Basically, the idea is to use multiple lasers (four in this case) to make multiple (four here) simultaneous images. This lets you look over a larger volume of tissue or with better temporal resolution. This microscope is a technological tour de force and effectively pushes the limits of this approach.

I am confident that there will be important special uses for this instrument. There are several limitations of the two-photon microscope, however, and this advance improves on one of them (making a larger or faster image), but not on the other (maximum depth in cortex that can be studied is only about 0.4 mm, whereas the cortex is at least 1 mm thick)....  Read more

Imaging calcium signals in neural structures like cortex are currently the only way to detect the activity of many or most neurons in a volume of tissue. Clay Reid in the Neurobiology Department at Harvard Medical School did this a while back and made important discoveries about the functioning of visual cortex. The current system, developed by Katsushi Arisaka, is a clever way to improve on the original method that Reid used. Basically, the idea is to use multiple lasers (four in this case) to make multiple (four here) simultaneous images. This lets you look over a larger volume of tissue or with better temporal resolution. This microscope is a technological tour de force and effectively pushes the limits of this approach.

I am confident that there will be important special uses for this instrument. There are several limitations of the two-photon microscope, however, and this advance improves on one of them (making a larger or faster image), but not on the other (maximum depth in cortex that can be studied is only about 0.4 mm, whereas the cortex is at least 1 mm thick). Furthermore, although the fourfold speed increase is impressive, something like a 10-fold increase—or 100-fold—is what you really would want. So this is a wonderful technological achievement that will be important, but it is not a "game-changer."

View all comments by Charles Stevens

This is lovely technology with promise for future important biological studies and represents one of a series of technical improvements in multiphoton microscopy that allow deep imaging (e.g., with GRIN lenses) or use of awake, behaving animals. Along with the exciting new opticogenetic reagents, we are a step closer to being able to use optical tools to monitor neuronal activity in populations of neurons during normal behaviors and under disease conditions.

View all comments by Bradley Hyman